Methods and mechanism for providing variable torque with variable torque resolution to rotation of a web roll

ABSTRACT

A mechanism (C) for tensioning a web extending from a rotating roll (R) includes three fluid relays ( 200 - 202 ) and two amplifiers ( 204, 205 ). A variable pressure pilot signal (P) drives a piston ( 66 ) of a rotational control apparatus (A) to create a pilot torque to the rotating roll (R). In the event that the variable pressure pilot signal (P) exceeds the bias pressures (B 1 -B 3 ) of the fluid relays ( 200 - 202 ), the pistons ( 64, 68, 70, 72  and  74 ) are driven by fluid relays ( 200 - 202 ) to create relay torques to the rotating roll (R) in addition to the pilot torque in direct proportion to differences between the variable pressure pilot signal (P) and the bias pressures (B 1 -B 3 ). The relay torques created by the second and third relays ( 201, 202 ) are amplified by the amplifiers ( 204, 205 ). Thus, the rotational control apparatus (A) can be fully utilized for its full torque range while still providing enough resolution to control tension of the web (W) precisely.

BACKGROUND

[0001] The present invention generally relates to methods and mechanisms for providing a torque output of a variable resolution, particularly to methods and mechanisms for controlling rotation with variable resolution, and specifically to methods and mechanisms for tensioning a web extending from a roll with variable resolution.

[0002] During web processing, it is necessary to properly tension the web as it is unwound or wound from a roll. The tension of the moving web is a critical parameter to control on a web machine. Improper tension can produce problems for downstream processes, causing poor print registration, inconsistent sheet length, poorly wound rolls, etc. Many methods are used to control the tension. One method is a pneumatic brake/clutch that creates a torque on the unwinding or rewinding roll. This torque generated by the brake/clutch translates directly into web tension. Controlling the tension then involves precisely controlling the torque on the unwinding or rewinding roll. On a typical web machine, the tension must be maintained to within +/−2%. Web machines such as flexographic printing machines may run total tension for a heavy grade paper at 222 N such that the tension tolerance of +/−2% translates to +/−3.61 Nm of torque for a roll having a diameter of 0.81 m and +/−0.395 Nm of torque when the roll unwinds down to the 0.089 m core. Likewise, at 222 N tension, the brake/clutch must generate 180 Nm of torque at the 0.81 m roll diameter versus 19.7 Nm of torque at 0.089 m roll diameter as represented by line Z of the graph of FIG. 1 where M represents roll diameter and NM represents torque. However, a typical brake/clutch that can generate 180 Nm of torque requires a large friction surface area and large pneumatic piston(s) and can not make torque changes of 0.395 Nm which is needed to maintain tension within the desired range when the roll diameter is 0.089 m. Thus, in order to maintain tension within the desired range, ever increasing torque resolution is needed as the torque (or roll diameter) decreases.

[0003] Further, many web machines run a variety of materials from very extensible polyethylene to non-extensible polypropylene and paper. These materials require very different tensions for printing, cutting or rewinding. As an example, the same flexographic printing machine running at 222 N for a heavy grade paper may run tensions of 35.6 N for a 2 mil polyethylene film. Thus, at 35.6 N tension, the brake/clutch must generate 28.9 Nm of torque at the 0.81 m roll diameter versus 3.16 Nm at the 0.089 m roll diameter as represented by the line V of the graph of FIG. 1. To maintain tension within the desired range, tolerance must be +/−0.578 Nm of torque for a roll having a diameter of 0.89 m and +/−0.063 Nm of torque for a roll having a diameter of 0.089 m. Thus, for the flexographic printing machine which can handle both heavy grade paper and polypropylene, the clutch/brake must have a torque resolution range of 3.62 Nm for heavy gauge paper at the 0.81 m roll diameter to 0.063 Nm for polypropylene at the 0.089 m roll diameter.

[0004] Assignee's U.S. Pat. No. 5,586,635 discloses a rotational control apparatus for applying varying degrees of torque by selectively driving pairs of pistons (which can be of the same or different areas). Although this clutch/brake goes a long way towards meeting this problem, a need continues to exist to selectively control driving the pistons automatically and in a bumpless fashion.

SUMMARY

[0005] The present invention solves this need and other problems in the field of tensioning of a moving web by providing, in the preferred form, a pilot torque to the rotation of a roll from which the web extends in direct proportion to a pilot signal and a first relay torque to the rotation of the roll from which the web extends in direct proportion to a difference between the pilot signal and a first, biasing level (which is greater than zero) whenever the pilot signal is greater than the first, biasing level, with the tension in the web created by the combination of the pilot and first relay torques. In most preferred aspects of the present invention, the first relay torque is in direct proportion to an amplitude (greater than one) of the first difference. In other preferred aspects of the present invention, second and third relay torques to the rotation of the roll are provided in direct proportion to differences between the pilot signal and second and third, biasing levels, which relay torques possibly being in direct proportion to amplitudes (greater than one) of the differences.

[0006] In other aspects of the present invention, rotation is controlled by applying a pilot torque to the rotation in direct proportion to a pilot signal and a first relay torque to the rotation in direct proportion to an amplitude (greater than one) of a difference between the pilot signal and a first, biasing level (which is greater than zero) whenever the pilot signal is greater than the first, biasing level.

[0007] In still other aspects of the present invention, rotation is controlled by applying a pilot torque in direct proportion to a pressure pilot signal and a relay torque in direct proportion to a fluid pressure signal provided by a fluid relay actuated by the pressure pilot signal and valving constant supply pressure between an inlet and an outlet in direct proportion to a difference between the pressure pilot signal and a biasing pressure (which is greater than zero).

[0008] It is thus an object of the present invention to provide novel mechanism and methods for controlling rotation.

[0009] It is further an object of the present invention to provide such novel mechanism and methods for controlling tension in a web extending from and rolled upon a rotating roll.

[0010] It is further an object of the present invention to provide such novel mechanism and methods providing variable resolution torque output.

[0011] It is further an object of the present invention to provide such novel mechanism and methods allowing automatic control.

[0012] It is further an object of the present invention to provide such novel mechanism and methods providing bumpless transitions.

[0013] It is further an object of the present invention to provide such novel mechanism and methods providing amplification to extend the range of resolution over the full torque range.

[0014] These and further objects and advantages of the present invention will become clearer in light of the following detailed description of an illustrative embodiment of this invention described in connection with the drawings.

DESCRIPTION OF THE DRAWINGS

[0015] The illustrative embodiment may best be described by reference to the accompanying drawings where:

[0016]FIG. 1 shows a graph representing torque requirements versus roll diameter for webs formed of different material.

[0017]FIG. 2 shows a mechanism for controlling tension of a material web on a roll according to the preferred teachings of the present invention.

[0018]FIG. 3 shows a graph representing total torque versus pilot signal for rotational apparatus of the mechanism of the type of FIG. 2 and for rotational apparatus not including the mechanism of the type of FIG. 2.

[0019] All figures are drawn for ease of explanation of the basic teachings of the present invention only; the extensions of the Figures with respect to number, position, relationship, and dimensions of the parts to form the preferred embodiment will be explained or will be within the skill of the art after the following description has been read and understood. Further, the exact dimensions and dimensional proportions to conform to specific force, weight, strength, and similar requirements will likewise be within the skill of the art after the following description has been read and understood.

[0020] Where used in the various figures of the drawings, the same numerals designate the same or similar parts. Furthermore, when the terms “first,” “second,” “third,” “area,” and similar terms are used herein, it should be understood that these terms have reference only to the structure shown in the drawings as it would appear to a person viewing the drawings and are utilized only to facilitate describing the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] A mechanism controlling tension of a material web W extending from a roll R utilizing methods according to the preferred teachings of the present invention is shown in the drawings and generally designated C. In the most preferred embodiment of the present invention, mechanism C utilizes a rotational control apparatus A of the type shown and described in U.S. Pat. No. 5,586,635. For purposes of explanation of the basic teachings of the present invention, the same numerals designate the same or similar parts in the present figures and the figures of U.S. Pat. No. 5,586,635. The description of the common numerals and rotational control apparatus A may be found herein and in U.S. Pat. No. 5,586,635, which is hereby incorporated herein by reference.

[0022] Rotational control apparatus A generally includes a housing 10 in which a center shaft or hub 26 is rotatably mounted. The core of roll R is suitably secured to hub 26 for relative rotation therewith. Rotation of hub 26 and roll R relative to the input/output is controlled by driving pistons 64, 66, 68, 70, 72 and 74 and specifically in the most preferred form utilizing fluid pressure. Although in the preferred form shown, pistons 64, 66, 68, 70, 72 and 74 are shown of equal cross sectional area, pistons 64, 66, 68, 70, 72 and 74 having different cross sectional relationships including but not limited to that as described in U.S. Pat. No. 5,586,635 can be utilized according to the teachings of the present invention. Likewise, pistons 64, 66, 68, 70, 72 and 74 can be driven in other manners such as electrically or combinations thereof according to the teachings of the present invention. Furthermore, although apparatus A is shown as being of the type as shown and described in U.S. Pat. No. 5,586,635, apparatus A can be of other types and/or configurations including but not limited to individual units attached to hub 26 and roll R in a serial or parallel manner, which would not obtain all of the advantages of the type shown in U.S. Pat. No. 5,586,635 such as but not limited to compactness but which would avoid some of the other concerns such as but not limited to diametric balance.

[0023] The controls of the web machine in which mechanism C is utilized generate a pilot signal P which is variable according to the amount of tension desired. As an example, pilot signal P in the preferred form is fluid pressure, with increasing tension requirements resulting in pilot signal P of increasing fluid pressure. However, it can be appreciated that pilot signal P can be generated in forms other than pressure including but not limited to electrical signals. Mechanism C according to the preferred teachings of the present invention receives variable pilot signal P and selectively drives and particularly provides fluid pressure to pistons 64, 66, 68, 70, 72 and 74 utilizing pilot signal P in the most preferred form of variable fluid pressure. Pilot signal P is variable according to an automatic control of the web machine, with pilot signal P being in the range of 0-414 Kpa for the torque being required in the web machine in the following description. Pilot signal P is connected to and drives piston 66 in the preferred form so as to provide a pilot torque to the rotation of roll R in direct proportion to variable pilot signal P.

[0024] Mechanism C according to the preferred teachings of the present invention includes first, second and third fluid relays 200, 201 and 202. The actuators of each of the fluid relays 200, 201, and 202 also receive pilot signal P, with the actuators controlling a pressure valve having an inlet connected to a constant supply pressure S such as 414 Kpa in the preferred form. The outlet of fluid relay 200 is connected to and drives piston 64 in the preferred form which is diametrically opposite to piston 66. The output of fluid relay 201 is connected to and drives pistons 68 and 70 in the preferred form which are diametrically opposite to each other. Likewise, the output of fluid relay 202 is connected to and drives pistons 72 and 74 in the preferred form which are diametrically opposite to each other.

[0025] In the most preferred form, the output of fluid relay 201 is connected to pistons 68 and 70 through a pressure amplifier 204, which amplifies the relay signal by a multiple greater than one and in the most preferred form doubles the fluid pressure from the output of fluid relay 201 to pistons 68 and 70. Likewise, the output of fluid relay 202 is connected to pistons 72 and 74 through a pressure amplifier 205, which amplifies the relay signal by a multiple greater than one and in the preferred form at a different degree than amplifier 204 and in the most preferred form triples the fluid pressure from the output of fluid relay 202 to pistons 72 and 74.

[0026] Fluid relays 200-202 each have a bias level or pressure which must be overcome before opening the pressure valve and then opens the pressure valve above the bias pressure in direct proportion to pilot signal P. Thus, fluid relay 200 has a bias pressure B1 which is greater than 0 and less than pilot signal P and in the preferred form is 103 Kpa. Fluid relay 201 has a bias pressure B2 which is different from and particularly greater than bias pressure B1 of fluid relay 200 and less than pilot signal P and in the preferred form is 206 Kpa. Fluid relay 202 has a bias pressure B3 which is different than bias pressures B1 and B2 and particularly greater than bias pressure B2 of fluid relay 201 and less than pilot signal P and in the preferred form is 276 Kpa.

[0027] Now that the basic construction of mechanism C according to the preferred teachings of the present invention has been set forth, preferred methods of operation and advantages of mechanism C can be explained. For purpose of explanation, it will be assumed that mechanism C was operating with no torque to the rotation of roll R and increasing torque being required. Specifically, in the preferred form, as pilot signal P increases from 0 to bias pressure B1, pilot signal P drives piston 66 which results in increasing pilot torque to rotation of roll R as represented by T1 in the graph of FIG. 3. As pilot signal P is less than bias pressures B1, B2, and B3 of relays 200-202, relays 200-202 are not actuated and fluid pressure is not communicated to and does not drive pistons 64, 68, 70, 72 and 74. As only piston 66 is driven and specifically pistons 64, 68, 70, 72 and 74 are not driven, torque TI is less than torque TC which would be produced if all pistons 64, 66, 68, 70, 72 and 74 were driven by pilot signal P or if apparatus A is utilized having an enlarged frictional surface area corresponding to the sum of pistons 64, 66, 68, 70, 72, and 74.

[0028] If the web processing machine requires further torque as represented by increasing pilot signal P, as pilot signal P increases from bias pressure B1 to bias pressure B2, pilot signal P drives piston 66 to provide or create a pilot torque to the rotation of roll R directly related to or in direct proportion to pilot signal P and the area of piston 66 plus relay 200 is actuated so that piston 64 is driven to provide a first relay torque to the rotation of roll R directly related to or indirect proportion to the area of piston 64 and the difference between pilot signal P and bias level or pressure B1 which results in increasing torque as represented by T2 in the graph of FIG. 3. As pilot signal P is less than bias levels or pressures B2 and B3 of relays 201 and 202, relays 201 and 202 are not actuated so that fluid pressure is not communicated to and does not drive pistons 68, 70, 72 and 74. As only piston 66 is driven directly by pilot signal P and piston 64 is driven related to pilot signal P less bias pressure B1 and specifically pistons 68, 70, 72 and 74 are not driven, torque T2 is less than torque TC.

[0029] If the web processing machine requires further torque as represented by increasing pilot signal P, as pilot signal P increases from bias level or bias pressure B2 to bias level or bias pressure B3, pilot signal P drives piston 66 to provide the pilot torque to the rotation of roll R directly related to pilot signal P and the area of piston 66, piston 64 is driven to provide a first relay torque to the rotation of roll R directly related to the area of piston 64 and the difference between pilot signal P and bias pressure B1 plus relay 201 is actuated so that pistons 68 and 70 are driven to provide a second relay torque to the rotation of roll R directly related to or indirect proportion of the areas of piston 68 and 70 and the difference between pilot signal P and bias pressure B2 which results in increasing torque to the rotation of roll R as represented by T3N in the graph of FIG. 3 if amplification is not provided and as represented by T3A in the graph of FIG. 3 if amplifier 204 of the preferred form is provided to amplify the second relay torque. As pilot signal P is less than bias pressure B3 of relay 202, fluid relay 202 is not actuated so that fluid pressure is not communicated to and does not drive pistons 72 and 74. As only piston 66 is driven directly by pilot signal P, piston 64 is driven related to pilot signal P less bias pressure B1, and pistons 70 and 72 are driven related to pilot signal P less bias pressure B2, torques T3N and T3A are less than torque TC.

[0030] If the web processing machine requires further torque as represented by increasing pilot signal P, as pilot signal P increases above bias level or bias pressure B3, pilot signal P drives piston 66 to provide the pilot torque to the rotation of roll R directly related to pilot signal P and the area of piston 66, piston 64 is driven to provide torque to the rotation of roll R directly related to the area of piston 64 and the difference between pilot signal P and bias pressure B1, pistons 68 and 70 are driven to provide the second relay torque to the rotation of roll R directly related to the areas of piston 68 and 70 and the difference between pilot signal P and bias pressure B2 plus relay 202 is actuated so that pistons 72 and 74 are driven to provide or create a third relay torque to the rotation of roll R directly related to or indirect proportion to the areas of pistons 72 and 74 and the difference between pilot signal P and bias pressure B3 which results in increasing torque to the rotation of roll R as represented by T4N in the graph of FIG. 3 if amplification is not provided and as represented by T4A in the graph of FIG. 3 if amplifiers 204 and 205 of the preferred form are provided to amplify the relay torques.

[0031] It should be appreciated that at maximum pilot signal P indicating full torque requirements, torque T4A is essentially equal to torque TC and particularly is slightly less in that piston 64 is driven directly related to the area of piston 64 and the difference between pilot signal P and bias pressure B1 and is not amplified. However, as shown by FIG. 3, mechanism C of the present invention provides high torque resolution for pilot signal P less than bias pressure B2 where torque requirements of the web processing machine are relatively low and provides lower torque resolutions for pilot signal P greater than bias level or pressure B2 where torque requirements of the web processing machine are high. Without amplifiers 204 and 205, pistons 68, 70, 72 and 74 are not fully utilized because of bias pressures B2 and B2 of relays 201 and 202. Amplifiers 204 and 205 allow apparatus A to have the same operating range as when mechanism C is not utilized but provides torque resolution over a greater portion of the operating range.

[0032] Although described with reference to increasing torque requirements, operation is essentially the same but reversed for decreasing torque requirements such as when roll R of heavy grade paper is unrolled from its maximum diameter to its minimum diameter. Specifically, all pistons 64, 66, 68, 70, 72 and 74 are engaged when pilot signal or pressure P is greater than bias pressure B3, pistons 72 and 74 are disengaged when bias pressure B3 is reached, pistons 68 and 70 are disengaged when bias pressure B2 is reached, and additionally piston 64 is disengaged when bias pressure B1 is reached, with only piston 66 being driven when pilot signal P is less than bias pressure B1 but greater than 0.

[0033] It should be appreciated that pistons 64, 66, 68, 70, 72 and 74 are engaged or disengaged in a bumpless or smooth transition utilizing mechanism C of the present invention in comparison to transition as would occur if pistons 64, 66, 68, 70, 72 and 74 would be engaged at pilot signal P such as through the use of a solenoid valve in the case of fluid actuation. Additionally, control of pistons 64, 66, 68, 70, 72 and 74 is automated and results from pilot signal P without further intervention utilizing mechanism C of the present invention.

[0034] It should be appreciated that pilot signal P could be created from constant supply pressure S in mechanism C of the present invention. Specifically, a suitable regulator or other control could create pilot signal P as a function of the torque requirements of the web processing machine by valving or otherwise releasing constant supply pressure S.

[0035] As discussed hereinbefore, variables of mechanism C including but not limited to the sizes and relationships between pistons 64, 66, 68, 70, 72 and 74, bias levels or pressures B1, B2, and B3 and the existence and amount of amplification provided by amplifiers 204 and 205 can be selected and adjusted according to the parameters of the particular application according to the teachings of the present invention. Similarly, apparatus A, relays 200-202 and amplifiers 204 and 205 of other types and configurations can be selected, adjusted and utilized according to the teachings of the present invention.

[0036] In the event that amplification is not desired, relays 200-202 could be integrally formed with rotational control apparatus A according to the teachings of the present invention. As an example, pistons 64, 68, 70, 72 and 74 could incorporate a spring bias that must be overcome before driving would occur.

[0037] Thus, since the invention disclosed herein may be embodied in other specific forms without departing from the spirit or general characteristics thereof, some of which forms have been indicated, the embodiments described herein are to be considered in all respects illustrative and not restrictive. The scope of the invention is to be indicated by the appended claims, rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein. 

1. Method for controlling rotation comprising: receiving a variable pilot signal and providing a pilot torque to the rotation in direct proportion to the variable pilot signal; actuating a first relay when the variable pilot signal is above a first level and providing a first relay torque to the rotation in addition to the pilot torque, with the first relay torque being in direct proportion to a first difference between the variable pilot signal and the first level; and amplifying the first relay torque provided by the first relay.
 2. The method of claim 1 further comprising: actuating a second relay when the variable pilot signal is above a second level and providing a second relay torque to the rotation in addition to the pilot torque and the first relay torque, with the second relay torque being in direct proportion to a second difference between the variable pilot signal and the second level.
 3. The method of claim 2 further comprising: amplifying the second relay torque provided by the second relay, with the first level being different than the second level, with the second relay torque being amplified at a different degree than the first relay torque.
 4. The method of claim 3 further comprising: actuating a third relay when the variable pilot signal is above a third level and providing a third relay torque to the rotation in addition to the pilot torque, the first relay torque, and the second relay torque, with the third relay torque being in direct proportion to a third difference between the variable pilot signal and the third level, with the third level being less than the first and second levels.
 5. The method of claim 4 with receiving the variable pilot signal comprising receiving the variable pilot signal in the form of a variable pressure pilot signal.
 6. Mechanism for controlling rotation comprising, in combination: means for receiving a variable pilot signal, with the variable pilot signal adapted to cause a pilot torque to the rotation in direct proportion to the variable pilot signal; a first relay receiving the variable pilot signal and creating a first relay signal in direct proportion to a first difference between the variable pilot signal and a first level, with the first relay signal adapted to cause a first relay torque to the rotation in addition to the pilot torque, with the first relay torque being in direct proportion to the first difference; and a first amplifier amplifying the first relay signal.
 7. The mechanism of claim 6 further comprising, in combination: a second relay receiving the variable pilot signal and creating a second relay signal in direct proportion to a second difference between the variable pilot signal and a second level, with the second relay signal adapted to cause a second relay torque to the rotation in addition to the pilot torque and the first relay torque, with the second relay torque being in direct proportion to the second difference.
 8. The mechanism of claim 7 further comprising, in combination: a second amplifier amplifying the second relay signal, with the first level being different than the first level, with the second relay signal being amplified by the second amplifier at a different degree than the first relay signal is amplified by the first amplifier.
 9. The mechanism of claim 8 further comprising, in combination: a third relay receiving the variable pilot signal and creating a third relay signal in direct proportion to a third difference between the variable pilot signal and a third level, with the third relay signal adapted to cause a third relay torque to the rotation in addition to the pilot torque, the first relay torque and the second relay torque, with the third relay torque being in direct proportion to the third difference, with the third level being less than the first and second levels.
 10. The mechanism of claim 9 with the receiving means comprising means for receiving a variable pressure pilot signal.
 11. Method for tensioning a web comprising: rotating a roll of the web; applying a pilot torque to the rotation of the roll in direct proportion to a variable pilot signal to create tension in the web extending from the roll; and applying a first torque to the rotation of the roll in addition to the pilot torque, with the first torque being in direct proportion to a first difference between the variable pilot signal and a first level, with the first level being greater than zero, with applying the first torque comprising automatically applying the first torque to the rotation of the roll whenever the variable pilot signal is greater than the first level, with the tension in the web extending from the roll being the combination of the pilot torque and the first torque.
 12. The method of claim 11 with the applying the first torque comprising applying the first torque to the rotation of the roll in direct proportion to a first amplitude of the first difference, with the first amplitude being greater than one.
 13. The method of claim 12 further comprising: applying a second torque to the rotation of the roll in addition to the pilot torque and the first torque, with the second torque being in direct proportion to a second difference between the variable pilot signal and a second level, with the second level being greater than zero, with the tension in the web extending from the roll being the combination of the pilot torque, the first torque and the second torque.
 14. The method of claim 13 with applying the second torque comprising applying the second torque to the rotation of the roll in direct proportion to a second amplitude of the second difference, with the second amplitude being greater than one, with the second amplitude being different than the first amplitude.
 15. The method of claim 14 with the second level being different than the first level.
 16. The method of claim 15 further comprising: applying a third torque to the rotation of the roll in addition to the pilot torque, the first torque and the second torque, with the third torque being in direct proportion to a third difference between the variable pilot signal and a third level, with the third level being greater than zero, with the tension in the web extending from the roll being the combination of the pilot torque, the first torque, the second torque and the third torque.
 17. The method of claim 16 with the third level being different from the second level and from the first level.
 18. The method of claim 13 with applying the second torque comprising applying the second torque to the rotation of the roll in direct proportion to a second amplitude of the second difference, with the second amplitude being greater than one, with the second level being different than the first level.
 19. The method of claim 11 further comprising: applying a second torque to the rotation of the roll in addition to the pilot torque and the first torque, with the second torque being in direct proportion to a second difference between the variable pilot signal and a second level, with the second level being greater than zero, with the tension in the web extending from the roll being the combination of the pilot torque, the first torque and the second torque.
 20. The method of claim 19 further comprising: applying a third torque to the rotation of the roll in addition to the pilot torque, the first torque and the second torque, with the third torque being in direct proportion to a third difference between the variable pilot signal and a third level, with the third level being greater than zero, with the tension in the web extending from the roll being the combination of the pilot torque, the first torque, the second torque and the third torque.
 21. Mechanism for tensioning a web extending from a rotating roll comprising, in combination: means for receiving a variable pressure pilot signal, with the variable pressure pilot signal adapted to cause a pilot torque to the rotating roll in direct proportion to the variable pressure pilot signal; and a fluid relay having an actuator, an inlet and an outlet, with the actuator receiving the variable pressure pilot signal, with the inlet being connected to a constant supply pressure, with the outlet adapted to cause a first relay torque to the rotating roll in addition to the pilot torque, with the first relay torque being in direct proportion to an outlet pressure, with the outlet pressure of the fluid relay being related to a difference between the variable pressure pilot signal and a first pressure, with the first pressure being greater than zero. 